From Western Regional Climate Center

Aberdeen
June 66/49 2.66 in
Sept 69/49 3.50 in

Astoria
June 64/50 2.50
Sept 68/50 2.59

Coos Bay
June 64/50 1.66
Sept 67/50 1.89

Eureka
June 61/51 0.63
Sept 63/51 0.73

So it is a little warmer in September and a little dryer in June.
However, early September is dryer and warmer than late September.
So I you were to start right after Labor Day -
You would have less traffic, warmer days, and dryer conditions.

The only advantage to June is longer days.